1. Technical Field
This invention relates to curable liquid silicone compositions, electrically conductive elastomers made from such compositions, and methods for the manufacture and use thereof.
2. Brief Description of the Related Art
Miniaturization and multi-functionality in electronic devices increasingly requires relatively thin (less than about 2.5 millimeters), soft (Shore A durometer less than or equal to about 60) gasketing materials for use on lightweight plastic components. This is true especially in applications such as, for example, cellular phones and computers. Federal and international regulations also place rigorous limits on how much electromagnetic radiation an electronic device can emit, as well as how immune the device must be to external electromagnetic interference (EMI), increasing the importance of EMI shielding in such devices.
Current approaches to gasket materials capable of EMI shielding include, for example, beryllium-copper finger stock, metal foil or metallized fabric wrapped around non-conductive foam gaskets (hereinafter FOF), non-conductive gaskets coated with conductive materials, highly filled-expanded polytetrafluoroethylene (hereinafter PTFE), and metal-based fillers loaded into silicone resins. However, these approaches lack the combination of effective EMI shielding along with the required softness and ability to be formed into thin cross sections. For example, FOF gaskets are soft and highly compressible, but are not readily wrapped with metallized fabric into complex shapes or shapes having thin cross-sections (e.g., less than about 760 micrometers, 30 mils), without leaving gaps. Filled, expanded PTFE compositions are soft, but lack physical strength, high electrical conductivity, and adequate compression set resistance.
Commercially available conductive silicone gaskets are typically gum-stock rubber based silicones compounded with conductive fillers by solid milling processes, and formed by molding. They are typically hard, having a Shore A Durometer greater than about 60. These gaskets accordingly provide poor cushioning and require high closure forces, which can cause damage to the plastic enclosure, or can require more complex assembly, creating more sites for radiation leakage. They also have poor compression set resistance values, generally greater than about 30% as per ASTM D395, Method B. This is a significant disadvantage in gasketing applications because a set in the material can cause a gap between the enclosure and the gasket, hence decreasing the shielding effectiveness.
Addition cure silicone compositions, known as “two-part” silicone compositions, are typically formed by the reaction of (a) an organopolysiloxane having at least two alkenyl groups per molecule (i.e., carbon-carbon double bonds), with (b) an organopolysiloxane having at least two silicon-bonded hydrogen atoms, in the presence of a metallic catalyst such as platinum. Electrically conductive addition cure silicone compositions have been described, for example, in U.S. Pat. No. 5,932,145 to Mitani et al., U.S. Pat. No. 6,017,587 to Kleyer et al., European Patent No. 0839870, European Patent No. 0971367, and Japanese Patent No. 63117065. These patents disclose that improved electrical conductivity in addition cure silicone compositions may be obtained by the addition of a non-reactive, volatile solvent to the uncured composition containing electrically conductive fillers, followed by removal of the solvent after cure to affect a volumetric shrinkage of the cured silicone. Similarly, U.S. Pat. No. 4,545,914 to Graiver et al. is directed to using water as a diluent, and European Patent No. 0 319828 uses volatile polydimethylsiloxane as a diluent that is eventually removed.
However, such conductive, cured silicone compositions possess low conductivity/high volume resistivity values (e.g., volume resistivity higher than about 1,000,000 ohm-centimeters), cannot be processed by liquid casting, and have poor physical properties. In particular, the cured compositions do not possess adequate softness, which is defined herein as Shore A durometer hardness of less than or equal to about 60, according to ASTMD-2240. Accordingly, there remains a need in the art for silicone elastomers that are highly electrically conductive and hence, EMI shielding, as well as soft, with good compression set resistance, and that can be made in thin cross sections, preferably through liquid casting process.